The present invention relates to a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate;
at least one chamber enclosing at least part of at least one of the radiation system, the patterning means, the substrate and the projection system, and
a contaminant trap arranged in the path of the projection beam.
The invention also relates to a method for manufacturing devices, which method uses the apparatus.
The term xe2x80x9cpatterning meansxe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
a programmable mirror array. An example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident radiation as deflected radiation, whereas non-addressed areas reflect incident radiation as non-deflected radiation. Using an appropriate filter, the said non-deflected radiation can be filtered out of the reflected beam, leaving only the deflected radiation behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC. This circuit pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
The term xe2x80x9cthe projection systemxe2x80x9d should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In order to be able to image smaller features, it has been proposed to use extreme ultraviolet radiation (EUV) with a wavelength in the range of 5 to 20 nm, particularly 13 nm, or a charged particle beam, e.g. an ion beam and an electron beams, as the exposure radiation in a lithographic apparatus. These types of radiation require that the beam path in the apparatus be evacuated to avoid beam scatter and absorption. Because there is no known material suitable for making a refractive optical element for EUV radiation, EUV lithographic apparatus must use mirrors in the radiation (illumination) and projection systems. Even multilayer mirrors for EUV radiation have relatively low reflectivity""s and are highly susceptible to contamination, further reducing their reflectivity""s and hence throughput of the apparatus. This imposes further requirements on the vacuum level that must be maintained and necessitates especially that hydrocarbon partial pressures be kept very low.
At the same time, plasma radiation sources and the resist are substantial sources of contaminants that must be kept out of the illumination and projection systems. A discharge plasma source, for example, uses a discharge to create very hot partially ionized plasma, which emits EUV radiation. The plasma gas, which is often xenon (Xe), and debris from the source must be kept from entering the illumination system. At the other end of the apparatus, the radiation incident on the resist to expose it causes ejection of debris and by-products by a sputtering process. It is necessary to prevent both source and resist debris from reaching the illumination and projection systems.
EP-A-0 957 402, incorporated herein by reference, discloses a system for preventing resist debris entering the projection system. This system comprises a simple tube, preferably conical, which surrounds the projector beam between the projection system enclosure and the substrate. A gas flow in the tube entrains resist debris to prevent it entering the projection system enclosure.
WO 99/42904, incorporated herein by reference, discloses a contaminant trap, called filter, for trapping source debris which comprises a plurality of foils or plates, which may be flat or conical and are arranged radial around the radiation source. The source, the filter and the projection system may be arranged in a buffer gas, for example krypton whose pressure is 0,5 Torr. The contaminant particles then take on the temperature of the buffer gas, for example room temperature thereby sufficient reducing the particles velocity before they enter the filter. The pressure in the known contamination trap is equal to that of its environment. This trap is arranged at 2 cm from the source and its plates have a length, in the propagation direction of the radiation, of at least 1 cm and preferably 7 cm. This design requires relative large and thus costly collecting and guiding/shaping optics to bundle and shape the source radiation and guide it to the mask.
It is an object of the present invention to provide an improved device for trapping debris, such as may be emitted by a plasma source or from resist exposed to EUV radiation.
This and other objects are achieved according to the invention in a lithographic apparatus as defined in the opening paragraph, which is characterized in that contaminant trap comprises at least two sets of channels arranged substantially parallel to the direction of propagation of said projection beam, which sets are spaced apart from each other along an optical axis of said projection beam, and means for supplying a flushing gas to the space between said two sets of channels.
A channel is understood to mean an elongated portion of space wherein a portion of the projection beam propagates. Plate members or foils separate such a channel from neighboring channels.
By providing two sets of channels, effectively forming two particle traps, and supplying gas to the space between them, a high gas pressure to trap the contaminant particles can be achieved, whilst the high flow resistance of the particle traps ensures that leakage is minimized. The new trap design allows the gas pressure in the trap to be orders of magnitude higher than outside the trap. The trap efficiency is increased considerably, which allows the trap to be considerably shorter, i.e. more compact so that the size and costs of the collecting optics can be decreased considerably. If sufficient exhaust pumping capacity is provided, the pressure outside the trap can be kept at least 10 or 100 times less than the pressure inside the trap.
In order to provide sufficient resistance to gas flow to maintain the pressure differential between the space between the two sets of channels and outside the trap, the channels defined by the plate member preferably have an aspect ratio (length/width) of greater than 8.
Preferably, the flow of gas through the traps is sufficiently fast that the volume of the trap is emptied by the exhaust pumps in less than, preferably less than half, the mean time taken for a gas molecule to diffuse through either one of the sets of channels. In this way, it is possible to ensure that contaminant particles that do not stick to the plate members or foils of the channels are entrained and exhausted with the flow of flushing gas.
The contaminant trap can be used between a radiation source and the illumination system comprised in said radiation system to capture source debris, especially where the source is a plasma source.
The invention also relates to a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
removing contaminant particles from the projection beam using a contaminant trap;
using patterning means to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material. This method is characterized in that the step of removing contaminant particles comprises sending the projection beam successively through at least two sets of channels, which are spaced apart from each other in the direction of propagation of the projection beam and supplying a flushing gas to the space between the said two sets of channels.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).